The present invention relates to a paper machine and in particular to an integrated paper machine having, in a machine running direction, a multi-layer head box, a gap former, a press section, a pre-dryer section, a dryer section and a closed web draw.
During the last 25 years, the increase in the consumption of paper and board was about 3.2% per year, and it is estimated that the increase will be about 2.8% per year during the following 15 years. Even though the total production of paper and board has increased constantly, the number of machines that produce these products has become lower. This is accounted for by the fact that new machines are always more efficient than their predecessors. A typical newsprint machine of the 1960""s produces about 100,000 tons of paper in a year, whereas a typical paper machine of the 1990""s produces about 300,000 tons of paper in a year. Such an increase in productivity has been based on constant product development in all of the constituent fields of papermaking.
The maximum width of a paper machine is these days about 10 meters, and it is not estimated that this width will be increased until the problems related to the massive constructions, such as high inertia, limitations related to production methods, and high costs have been solved. This will require considerable development in the fields of material sciences and production processes.
On the contrary, the running speed of the paper machine will still be increased. The speed has become three-fold in the course of the last 30 years, and no signs are seen regarding a slowing down of the increase in speed. The speed of current paper machines is about 1800 meters per minute, and speed records are made almost on a daily basis. The productivity will be increased by increasing the running speed of the machine in spite of the fact that the problems related to papermaking are increased almost exponentially along with an increase in the speed.
In the lecture xe2x80x9cFuture Paper Production Line Concepts Regarding Energy and Environmental Aspectsxe2x80x9d by Talja R., et al, Jun. 23 . . . 26, 1997, Baden-Baden, XXVI EUCEPA Conference, future perspectives of the development of paper machines are presented. In said paper, the applicant""s vision about a paper machine of a near future is presented, whose running speed is 2000 meters per minute. The paper machine comprises a multi-layer dilution headbox, a SpeedFormer gap former, a SymBelt shoe press, in which there is a closed transfer-belt transfer to the dryer section, a JetSym Run II dryer section, in which there are high-capacity drying units and conventional cylinder dryers, an on-line OptiLoad soft calender, and an OptiReel centre-drive reel-up.
The object of the present invention is to construct a paper machine by whose means it is possible to run at a speed higher than 2000 meters per minute and which is shorter than the present-day paper machines, whose overall length is about 130 meters. Further, as a raw-material in the paper machine, it must also be possible to use recycled fibre or some other fibre raw-material whose paper-technical properties are inferior.
By means of a paper machine in accordance with the present invention, it is possible to produce paper of good quality at a speed that is higher than 2000 meters per minute. The length of a paper machine in accordance with the present invention is shorter than the length of present-day paper machines.
The invention involves a number of features by whose means the speed capacity of the paper machine is affected. First, by means of a multi-layer headbox, within the scope of the quality-technical limits of the paper, it is possible to optimize the formation of stock components as layers so that a maximal water draining capacity is obtained in the former. A gap former provides a superb quality and runnability at high running speeds. Its problem is limited water draining capacity in particular with higher basis weights. A pre-press raises the dry solids content before transfer of the web to the press. This makes a pick-up transfer ever more reliable in particular at high speeds. When a major part of the water is already removed in the pre-press, the runnability of the press section proper is also improved. Traditionally, paper machines have an open draw from the press to the dryer section. Since the dry solids content of the web at the time of transfer from the press to the dryer section is about 50% or lower, the web is of low strength and susceptible of web breaks. The biggest problem of traditional cylinder drying is runnability on the first cylinders. The situation is deteriorated further by a closed transfer to the dryer section, because the web has not been tensioned in a draw of open transfer. A pre-drying unit fitted in accordance with the present invention of ours eliminates said problem as the unit dries the web further before the web enters into cylinder drying. A calender integrated in the dryer section permits calendering of the web so that the web is constantly supported. Of course, this also improves the runnability as it eliminates the fluttering and wrinkling of the web, which are typical of open gaps. In particular, a closed calender promotes threading of the web at high speed. The same arguments also apply to a closed draw to the reel-up.
In the solutions known from the prior art, just partial improvements have been suggested for different parts of the machine, but it is only by combining the above factors in one and the same concept that it is possible to put the advantages provided by said factors to use to full extent.
By means of a paper machine in accordance with the invention, it is also possible to have a positive effect on the quality of the paper. A multi-layer headbox provides abundant possibilities for optimizing the paper quality either by means of formation of fibre layers or by means of formation of layers of admixtures/chemicals in particular in combination with a roll-shoe gap former. In this way it is possible to apply a positive effect in particular on the printing quality of paper. As compared with other formers, a gap former provides by far the best paper (formation, uniformity of basis weight, orientation profile, distributions of fibres, fillers and fines in the direction of thickness, etc.), for which reason the gap former has largely replaced other former types in high-speed machines. If necessary, an extended-nip press provides economies in the bulk and porosity of the web. When the web does not have to be drawn between the press and the dryer section, the properties of strength of the paper are improved. Good runnability in the dryer section eliminates possible wrinkles and provides a more uniform tension profile. With a calender integrated in the dryer section, a higher drying capacity is obtained as compared with a dryer section with no pre-calendering. By means of pre-calendering, it is also possible to improve the properties of strength, smoothness and gloss of the paper. Impingement drying makes it possible to utilize drying that prevents shrinkage more efficiently than in the prior art and, thus, to provide, for example, a web whose dimensional stability in the cross direction is uniform. Formation of waves is also reduced.
The length of a paper machine is made shorter by the following features in accordance with the present invention:
pre-pressing removes an abundance of water right in the wire part, in which case, under favourable conditions, just one press nip is needed in the press section,
in the press section, in one embodiment, impulse drying or hot pressing is utilized, each of which increases the dry solids content after the press, which reduces the required need of drying,
in the beginning of the dryer section, there is an efficient pre-drying unit,
in the press section, the relative proportions of cylinders and suction rolls have been optimized,
in the dryer section, there is an impingement drying unit fitted below the floor level, whose drying capacity is multiple as compared with cylinder drying,
calendering in the dryer section improves the contact between the web and the cylinders, in which case the drying efficiency of the cylinders placed after the calender is increased,
in a paper machine in accordance with the invention, conventional long draws in connection with the calender are avoided.
It is an essential realization in the present invention that the overall result is not even nearly optimal if the improvements known from the prior art are carried into effect one by one. For example, the speed potential of the paper machine is not increased at all irrespective of how high the running speeds of the former and of the press are if the web does not run through the dryer section or through a finishing device. In a similar way, for example, by means of multiple-layer formation certain improvements are achieved in the properties of the paper, but it is the pre-calendering in the dryer section that provides maximal advantages.
The dry solids contents in a paper machine in accordance with the present invention are typically:
after the headbox, 0.3 . . . 3%,
before the press, 18 . . . 30%,
after the press, 45 . . . 65%, depending on the technique that is employed,
after the pre-dryer, 55 . . . 70%, and
after the dryer section, 92 . . . 98%, depending on paper grade.
In the following, components suitable for a paper machine in accordance with the invention will be described with reference to solutions in themselves known and suggested in patent literature or in other publications.
With respect to the multi-layer headbox suitable for the paper machine in accordance with the present invention, reference is made to the solution described in the applicant""s FI Patent Application 964704. What is concerned is a multi-layer headbox, which comprises stock inlet headers, a tube bank, intermediate chambers, a turbulence generator, and a slice cone. The multi-layer headbox further comprises means for supplying the dilution liquid to a distribution plate placed between the inlet header and the tube bank. In this headbox, the stocks pass separate from one another to the end of the slice cone, where they are finally combined when they are transferred to the web former. Thus, for this multi-layer headbox, the designation fibrelayer headbox can be used.
In a paper machine in accordance with the invention, it is also possible to use a multi-layer headbox described in the applicant""s FI Patent 92,729. In this solution, into each inlet header in the multi-layer headbox, a stock concept is supplied which has been produced out of the same fresh stock by adding to the fresh stock the necessary chemicals and fillers. For this multi-layer headbox, thus, the designation chemical/filler layer headbox can be used.
The consistency of the stock that is fed from the slice opening of the headbox of a paper machine in accordance with the invention into the gap former can be 1 . . . 3%, whereas a normal consistency of the stock is about 0.3 . . . 1%. The technique that can be concerned in web formation out of a high-consistency stock has been described, for example, in the paper xe2x80x9cHigh consistency sheet forming, Part 1: Research and development of headboxes, Part 2: Pilot plant test, and Part 3: Sheet quality and engineering dataxe2x80x9d by Tadayoshi Nomura et al, January 1989, Tappi Journal, pp. 115 . . . 122, 171 . . . 176, and 187 . . . 192. In this connection, reference is also made to the lecture xe2x80x9cNew Developments in High Consistency Formingxe2x80x9d by Sandgren B., SPCI conference on Jun. 4 . . . 7, 1996.
The scope of the embodiments of the present invention also includes a solution in which just a part of the stocks in a multi-layer headbox are at a higher consistency, as is suggested, for example, in the U.S. Pat. No 4,376,014.
In a paper machine in accordance with the invention, it is possible to use a roll-gap former, a shoe-gap former, or a roll-shoe gap former. One advantageous gap former suitable for a paper machine in accordance with the invention is the roll-shoe gap former marketed by the applicant with the product name SpeedFormer, in whose respect reference is made, for example, to the lecture xe2x80x9cBenchmarks in the Forming of Printing Gradesxe2x80x9d by Ahonen P., held at the conference Valmet Paper Machine Days on Jun. 13 . . . 14, 1996 (copy attached to the present patent application. The gap former may also be provided with water drain elements that can be loaded, of which one embodiment is described in the applicant""s FI Patent 98,540.
With respect to formers that can be applied in a paper machine in accordance with the invention, reference is also made to the U.S. Pat. No. 4,790,909 (Beloit Corp.), in which a roll-shoe gap former is described, to the U.S. Pat. No. 5,201,999 (Beloit Corp.), in which a shoe-gap former is described, and to the U.S. Pat. No. DE 4,117,597 (J. M. Voith GmbH), in which a gap former is also described.
In the U.S. Pat. No. 5,160,583 (Beloit Corp.), an integrated headbox/gap-former solution is described, in which the stock is constantly in a closed space when it moves from the headbox into the gap between the wires. In the solution, there is no free discharge jet and no free stock surface, which might be unstable and be decomposed in a detrimental way at high speeds. In a paper machine in accordance with the present invention, an integrated headbox/gap-former solution of this type can also be employed.
A second solution of integration of headbox and former which can be used favourably in a paper machine in accordance with the present invention has been described in the lecture xe2x80x9cThe Contro Flo Formerxe2x80x9d by Sorma O., TAPPI Engineering Conference 1983.
The gap formers mentioned above have been placed vertically so that the path of the stock is substantially in a vertical plane. In a paper machine in accordance with the invention, it is also possible to use a horizontal gap former, in which case the path of running of the stock is substantially horizontal.
In a paper machine in accordance with the invention, pre-pressing is employed in the former. The pre-pressing takes place preferably by means of an extended-nip press, but a roll nip can also be used. The shoe roll of the pre-press can be placed inside any of the two wire loops, depending on the details of the construction of the former and of the press section.
With respect to pre-pressing taking place by means of an extended nip, reference is made to the applicant""s FI Patent 96,789, in which a press section and a press stage accomplished in the former by means of an extended-nip press zone are described. In the extended-nip press zone, preferably a compression pressure of 0.5 . . . 3 MPa is employed, and the length of the extended-nip press zone is preferably in a range of 100 . . . 300 mm.
With respect to pre-pressing taking place by means of a roll nip, reference is made to the applicant""s FI Patent 98,843, in which the pre-press fabrics consist of a wire and of a transfer belt that does not receive water, and to the applicant""s U.S. Pat. No. 5,522,959, in which the fabrics of the pre-press placed in the end of the former consist of two wires or of a wire and a press felt.
In the U.S. Pat. No. 4,879,001 (Beloit Corp.), a gap former is described, in which there is a pre-press based on a roll nip and in which the press felt that runs through the pre-press runs further into the following shoe nip.
In a paper machine in accordance with the invention, a press section is used in which there is at least one extended-nip press.
With respect to the press section, reference is made to the applicant""s FI Patent 96,789 and to the prior-art publications referred to in said patent. In the patent, a press section of a paper machine is described, which comprises at least two separate press nip zones. At least through the first press nip zone, two press fabrics run which receive water, and the web runs between said fabrics through said nip zone. The paper web to be pressed has a closed draw supported by a press fabric from the pick-up point to the dryer section without free, unsupported draws.
In the applicant""s FI Patent 81,854, the idea is suggested that a transfer fabric that does not receive water is employed in the press. Also, in the U.S. Pat. No. 4,483, 745 (Beloit Corp.), a press section is described in which the web runs through the press section between a belt not receiving water and a press felt. The nip or nips in the press section can be extended nips or roll nips. In a paper machine of the invention, said transfer fabrics not receiving water can be used in a way in itself known.
With respect to the rolls employed in the press section in a paper machine in accordance with the invention, reference is made to the applicant""s U.S. Pat. No. 5,084,137 and to the applicant""s FI Patents 75,217 and 79,177. In these patent publications, various variable-crown or adjustable-crown roll solutions are described.
At the rolls in the press section, glide bearings can also be used. Solutions with glide bearings have been described in the applicant""s FI Patent 97,565 and in the applicant""s FI Patent Application 970624.
In a paper machine in accordance with the invention, it is also possible to use impulse drying in the press section.
With respect to impulse drying, reference is made to the applicant""s FI Patent 89,284, in which a method of impulse drying and an impulse dryer are described.
Impulse drying has also been described in the U.S. Pat. No. 4,324,613 (Wahren), in the WO Patent 95/10659 (Beloit Corp.), and in the EP Patent 296730 (Pulp and Paper Research Institute of Canada et al).
In stead of impulse drying, in the press section, it is also possible to employ hot pressing. Hot pressing is in the other respects similar to impulse drying, but in hot pressing lower temperatures are used. In hot pressing, the surface temperature of the backup roll is in a range of about 60 . . . 170xc2x0 C., and in impulse drying it is higher than 170xc2x0 C.
In a paper machine in accordance with the invention, a pre-dryer section is used, which is placed between the press section and the dryer section proper. The pre-dryer section can consist of an impingement-drying and/or through-drying cylinder of very large diameter and provided with a perforated mantle. The pre-dryer section can also consist of a substantially planar dryer section in which impingement drying is used, or of a dryer section of the Condebelt type.
With respect to a planar pre-dryer section provided with impingement drying, reference is made to the applicant""s FI Patent Application 954714. In said patent application, a pre-dryer section is described in which the web runs in a substantially horizontal plane between an upper and a lower drying wire and in which impingement drying is applied to the web through the upper wire by means of an upper impingement drying unit, and impingement drying is applied to the web through the lower wire by means of a lower impingement drying unit.
In the U.S. Pat. No. 4,361,466 (Beloit Corp.), a planar, slightly upwards curved, relatively long pre-dryer section is described. Here the web runs on a belt loop penetrable by air and supported from below by suction rolls. Above the web, there are impingement drying units, by whose means drying air is blown towards the web.
In Condebelt drying, the web and the drying felt that supports the web run between two metal bands which form loops. The metal band that is in contact with the web is heated in order to evaporate water from the web, and the metal band that is in contact with the drying felt is cooled in order to condense the water that is evaporated from the web into the drying felt.
With respect to Condebelt drying, reference is made to the applicant""s FI Patents 59,439, 80,102, and 96,790 and to the lecture given in the seminar at The Helsinki Symposium on Alternate Methods of Pulp and Paper drying, xe2x80x9cAn Assessment of the Quality Aspects of Condebelt Dried Board and Paperxe2x80x9d by Unkila K., et al, Tampella Papertech Oy.
The dryer section in a paper machine in accordance with the invention starts favourably with a normal cylinder drying group that makes use of single-wire draw, in which the drying cylinders are placed above and the reversing suction rolls below.
After the first cylinder drying group, in the dryer section, it is possible to use a drying unit described in the applicant""s FI Patent 100,013, which consists of a large-diameter impingement-drying and/or through-drying cylinder, at each of whose sides contact drying cylinders have been fitted. In the dryer section, there can be one or several such drying units. In this connection, reference is also made to the paper xe2x80x9cNeue Trockenpartiekonzeptef fxc3xcr Papiermaschinenxe2x80x9d (xe2x80x98New dryer section concepts for paper machinesxe2x80x99) by Yli-Kauppila J., Ilvesp{umlaut over (aa)} H., Das Papier, Heft 10A, 1995.
With respect to the location of a large-diameter impingement-drying and/or throughdrying cylinder, reference is also made to the applicant""s FL Patent Application 971713, in which the large-diameter impingement-drying and/or through-drying cylinder has been fitted in the space below the floor level of the paper machine hall and is provided with a blow hood which can be opened and closed.
With respect to impingement drying units, reference is made to the applicant""s FI Patent Application 951746, in which heated air is blown by means of an impingement drying unit installed above a drying cylinder through the drying wire against the web which runs on the face of the drying cylinder.
In impingement drying, hot air or superheated steam is employed. In the impingement drying hood, heating and blow means have been integrated, as is described in the applicant""s FI Patent Application 980766.
The final end of the dryer section in a paper machine in accordance with the invention consists of drying groups that make use of single-wire draw and in which groups the drying cylinders are placed above and the reversing suction rolls below.
In the dryer section, regulation of the curl of the paper web is also carried out. This is carried out in a way in itself known by means of steam boxes and/or by means of impingement drying units fitted in connection with drying cylinders.
In a paper machine in accordance with the invention, it is also possible to use a surface treatment section, e.g. a surface sizing and pigmenting device marketed by the applicant with the product name SymSizer.
In a paper machine in accordance with the invention, in the surface sizing equipment, it is also possible to use belt support, with respect to which reference is made to the applicant""s FI Patent Application 970387.
The curl of a paper web is controlled in the ways suggested, for example, in the applicant""s FI Patent Application 964830 or in publications referred to in said patent application. In the dryer section, the regulation of the curl has been accomplished by means of steam boxes, but instead of and/or in addition to said boxes, it is possible to use impingement drying boxes fitted on the drying cylinders in the dryer section.
In a paper machine in accordance with the invention, pre-calendering or any other pre-finishing is carried out in the dryer section. With respect to pre-calendering, reference is made to the applicant""s FI Patent Application 960925. Inside a drying group that makes use of normal single-wire draw or in connection with the last drying cylinder, a free space has been arranged, in which a calender roll has been fitted. Together with a drying cylinder or roll, the calender roll forms a calendering nip. The calender roll can also be an extended-nip roll.
The surface treatment mentioned above can also be carried out in the dryer section.
With respect to the reversing suction rolls used in the dryer section, reference can be made to the roll solution described in the applicant""s FI Patent 83,680. What is concerned is a roll marketed by the applicant with the product name VacRoll, which roll is provided with a perforated mantle and in which roll there is no separate suction box, but the whole of the interior space in the roll mantle is kept under vacuum. Of course, conventional suction rolls can also operate as reversing rolls.
In a paper machine in accordance with the invention, it is also possible to use an OptiSoft soft calender marketed by the applicant or an OptiLoad supercalender marketed by the applicant, which calender is described in the applicant""s FI Patent 96,334.
The transfer of the paper web from the dryer section to the calender has been accomplished preferably as a closed draw. In this respect, reference is made to the applicant""s FI Patent Application 973725, in which a closed draw from a drying cylinder of the dryer section to a supercalender is described.
In a paper machine in accordance with the invention, as the reel-up, it is possible to use an OptiReel reel-up marketed by the applicant, which reel-up is described in the applicant""s FI Patent 96,334.
The transfer of the paper web to the reel-up is preferably also accomplished as a closed draw. In the DE Patent 43 28 310 (Voith), a closed draw of the web to the reel-up has been described earlier.